The 22 micron emission feature in supernova remnants and massive ...

The Evolving ISM in the Milky Way & Nearby Galaxies
The 22 micron emission feature in supernova remnants and
massive star-forming regions
Takashi Onaka 1 , Thomas. L. Roellig 2 , Yoko Okada 3 , and Kin-Wing Chan 1,4
ABSTRACT
Spitzer/IRS observations have confirmed the presence of the broad emission
feature around 22µm in the Carina nebula suggested by ISO/SWS observations.
They have indicated that the feature peak is located around 23µm and the band
width/profile may vary in the nebula. It has been suggested that the feature
originates from dust grains formed in supernovae since a similar feature has also
been reported in Cas A SNR. IRS observations of Cas A confirm the feature, but
it seems to peak at a little shorter wavelength (about 21µm) and has a sharper
profile than that seen in the Carina nebula. IRS spectra of some HII regions
also show a broad emission feature similar to the Carina spectrum. These results
suggest that dust grains relating to SNe or massive star-formation have a feature
around 20–23µm, which may originate from dust grains with similar optical
characteristics. We suggest that the shape effect of surface modes of FeO grains
could account for the observed variety of the features if the shape distribution
evolves from a nearly mono-shape (spherical shape) distribution to a continuous
shape distribution possibly by coagulation. The understanding of the nature of
the feature is important for the investigation of recent star-formation activities
in external galaxies.
Subject headings: dust, extinction — supernovae: general – supernova remnant
— HII regions
1 Department of Astronomy, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan
2 NASA Ames Research Center, MS245-6, Moffet Field, CA94035-1000, U.S.A.
3 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Kanagawa 229-8520,
Japan
4 MaxEmil Photonics Corp., 6F, 131, Lane 235, Bao Chiao Rd, Hsintien City, Taipei, Taiwan

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1. Introduction
Supernovae (SNe) are thought to be a major dust supply source in galaxies. While
theoretical studies suggest efficient dust formation in SN ejecta (Kozasa et al. 1991; Todini
& Ferrera 2001; Nozawa et al. 2003), observational evidence for a significant amount of dust
grains that are formed in or associated with SNe has not yet been confirmed (e.g., Dunne et
al. 2003; Morgan et al. 2003; Krause et al. 2004; Sugerman et al. 2006; Meikle et al. 2007;
Sakon et al. 2007). It also remains uncertain observationally what kind of dust grains are
really formed in SNe.
Arendt et al. (1999) have detected a broad 22µm band emission in one of the knots
in Cas A and suggested that it originates from dust grains newly formed in the SN. Based
on ISO/SWS observations, Chan & Onaka (2000) reported the presence of a similar broad
emission feature in the Carina nebula, one of the most active star-forming regions in the
Galaxy. Since a similar feature is also detected in M17 (Jones et al. 1999), Chan & Onaka
(2000) suggest a possible link of this feature to dust grains formed in SNe. However the
SWS instrument had a change in the slit size within the feature and it is therefore difficult
to unambiguously confirm the presence of a broad feature and investigate the band profile
in the 20µm region for extended sources (Peeters et al. 2002).
As part of a GTO program (PI: T. L. Roellig) Spitzer/IRS observations of the Carina
nebula as well as supernova remnants and starburst galaxies have been carried out to further
examine the presence and the relation of the feature to supernova remnants (SNRs) and
massive star-forming regions. The IRS long-wavelength low-resolution module has the same
slit size for 14–38µm and thus is suitable to quantitative investigation of a broad feature
in the 20µm region. Its long slit also enables us to examine the spatial distribution of the
feature efficiently. The presence of a broad feature around 23µm is confirmed in the IRS
spectrum of the Carina nebula (Onaka et al. 2008). This report investigates the features
seen in the Carina feature in relation to those of Cas A and other HII regions and discusses
possible band carriers.
2. Observations and Results
The Carina nebula was observed with the IRS long-low module in the spectral mapping
mode. Details of the IRS observations of the Carina nebula are given in Onaka et al. (2008).
Although the signal was saturated at the position where the SWS observation detected the
22µm feature, a broad feature around 23µm was clearly detected in the ionized region, but
the feature was not seen in the molecular cloud region (Fig. 1). The feature appears to be

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the strongest at the ionization front (position B), the Carina I-E complex (Brooks et al.
2003). The origin of the difference in the peak position between the IRS spectrum and the
SWS spectrum is not clear since the IRS spectra at the SWS position were saturated and
no useful spectra were extracted.
µ
Fig. 1.— Long-low IRS spectra of the Carina nebula. The black line (position A) indicates the
IRS spectrum in the ionized region, the red line that near the Car I-E radio complex (position B),
and the green line that in the molecular region (position C; see Onaka et al. (2008) for the accurate
position). The SWS spectrum is also shown for comparison by the blue line, which is scaled to
match with the IRS spectrum at position B.
µ
µ
Fig. 2.— IRS spectra taken with the long high-resolution module. (a) Sharpless 171 (b) G333.6-
0.2. See Okada et al. (2008) for details of the observations. The red lines indicate the assumed
continua.
IRS spectra at several positions in Cas A clearly show a broad 22µm feature (Ryo et al.
2008). We have also found that similar features are seen in IRS high-resolution spectra of
HII regions (Okada et al. 2007). Fig. 2 shows some examples of HII region spectra, whereas

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comparison of the features in these objects is shown in Fig. 3, where continua have been
subtracted from the spectra and the profiles are normalized at the peak. The band feature
of G333.6-0.2 is quite similar to that of Sharpless 171 (S171) and is not shown. It is clearly
seen that the Cas A feature peaks around 21µm, which is distinctly shorter than the Carina
and S171 features. The feature at position A is sharper, while that at position B seems to
be similar to the S171 feature.
µ
Fig. 3.— Comparison of features around 22µm in various objects. The red line indicates
the Cas A feature, the black line that of the position A of the Carina nebula, the green line
that of postion B, and the blue line the feature in the Sharpless 171 spectrum.
3. Candidates for the Feature Carrier
Several candidate materials have been proposed for the 22µm band carriers, including
FeO, proto-silicates, and TiO (cf., Onaka et al. 2008). The present investigation suggests
that the features seen in Cas A and HII regions are similar but not exactly the same. In the
following we investigate a single carrier material that may account for the observed variety
of the 22µm feature by the particle shape effect.
According to Bohren & Huffman (1983), small particles have surface resonance modes
in the spectral range where the real part of the dielectric constants becomes negative and
the peak and profile of the resonance band feature depend sensitively on the particle shape
or shape distribution. Fig. 4a shows the dielectric constants of FeO (Henning & Mutschke
1997), indicating that the surface resonance occurs in the spectral region from 20 to 30µm.

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Fig. 4b indicates the calculated emissivity of FeO particles of the various shape distributions.
Spherical FeO particles (blue dotted line) have a sharp emissivity peaking around 20µm.
FeO particles with a continuous distribution of ellipsoids (uniform CDE; Borhen & Huffman
1983) are indicated by the solid black line, which shows a very broad feature, while those
with the continuous distribution weighted on the spherical shape (red dashed line) have an
intermediate profile (weighted CDE; Ossenkopf et al. 1992).
ε
ε
ε
ε
µ
µ
Fig. 4.— (a) Dielectric constants of FeO (Henning & Mutschke 1997). The shaded area indicates
the region where the real part (ǫ 1 ; black solid line) becomes negative and where surface modes can
occur. The imaginary part (ǫ 2 ) is indicated by the red dotted line. (b) Emissivity of FeO of various
shape distributions. The emissivity of a sphere is indicated by the blue dotted line, that of the
weighted CDE by the red dashed line, and that of the uniform CDE by the black solid line. All
the curves are normalized by their peak values.
Fig. 5 shows comparison of the observed features with those calculations. A single
temperature is assumed for FeO particles and a smooth continuum is added to fit the observed
spectra. Either FeO spheres or FeO with the weighted CDE can account for the sharp
feature around 21µm seen in Cas A. The sharp feature around 23µm at position B of
the Carina nebula may be matched with the emissivity of FeO particles of the weighted
CDE, but FeO spheres have a peak at too short a wavelength. The broad features seen at
position B of the Carina nebula and in HII regions can be accounted for by FeO of CDEs.
Because the CDE is rather an arbitrary assumption and the actual shape distribution could
be different, perfect match should not be expected. These investigations suggest that the
particle shape effect could account for the observed variety of the 22µm feature if it originates
from surface modes of small particles. It is also suggested by Bohren & Huffman (1983) that
coagulation of particles makes the emissivity similar to those with CDEs. Thus the observed
variations in the 22µm feature may be interpreted in terms of the evolution of the particle
shape distribution; they may start with being spherical or a nearly mono-shape distribution,

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showing a narrow feature, when they are formed in SNRs. As they spend their time in
HII regions, they get coagulated, for which the shape distribution becomes close to a CDE.
FeO is a candidate material that has surface modes in the spectral range in question, but
any materials that have similar optical characteristics (negative real part of the dielectric
constants in 20–30µm) could account for the observed variation.
µ
µ
Fig. 5.— Comparison of the 22µm features with FeO grains of various shape distributions. (a)
Cas A, (b) position A of the Carina nebula, (c) position B of the Carina nebula, and (d) Sharpless
171. The observed spectra are shown by the thin black solid lines. Fits with FeO sherical particles
are shown by the blue dotted lines. FeO grains of the weighted CDE are indicated by the red
dashed lines, whereas those of the uniform CDE are shown by the thick black solid lines.
4. Summary
Spitzer IRS spectra of Cas A SNR, the Carina nebula, and some HII regions confirm
the presence of broad features in 21–23µm. This feature is broader than “21µm” feature,
whose actual peak is at 20.1µm, seen in carbon-rich post-asymptotic branch stars (Volk et

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al. 1999), and is different. The present results reveal the variety of the 22µm feature. It is
sharp and peaks at a shorter wavelength (∼ 21µm) in Cas A, while spectra of HII regions,
including that at position B of the Carina nebula, are broad and peak at a longer wavelength
(∼ 23µm). The spectrum of position A of the Carina nebula has a sharp profile peaking
around 23µm.
The observed spectral variety can be interpreted in terms of the shape effect if the
feature originates from surface modes of small particles. FeO particles of various shape
distributions can account for the observed variety of the features to some extent, although
they do not provide a perfect match. It is difficult to estimate the actual shape distribution
of dust grains. The observed variation may be understood if the particle shape distribution
evolves from a nearly mono-shape (spherical) at the beginning of dust formation and then
becomes broad when the particles start to coagulate.
The present investigations suggest that the 22µm feature is associated with massive
star-forming regions and possibly originates from dust grains formed in SNe. Some starburst
galaxies indicate a similar feature around 22µm (Tajiri et al. 2008), suggesting that this
feature can be used for diagnosis of recent activities in distant galaxies and could impact on
the number count in deep surveys (Pearson et al. 2004).
This work is based on observations made with the Spitzer Space Telescope, which is
operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology under
a contract with NASA and is supported by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology, Japan and that from the
Japan Society for the Promotion of Science.
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This preprint was prepared with the AAS L A TEX macros v5.2.